1. Field of the Invention
The present invention relates to position control systems of actuators with brush-type permanent magnet DC motors and, more particularly, to actuator position control systems utilizing commutation pulse feedback.
2. Description of the Related Art
The feedback signals of actuator position control systems are normally achieved by using encoders, resolvers, or potentiometers. More recently, commutation pulses embedded in the current of brush-type permanent magnet DC motors have been used as feedback signals (referred to as “pulse count”).
Encoders and resolvers can provide good accuracy and reliability but they are too expensive for applications like HVAC control in automobiles. Potentiometers have been a popular choice for such low cost applications.
The typical arrangement for a potentiometer actuator includes a brush-type permanent magnet DC motor, and a gear train to increase the output torque. The potentiometer is driven either directly by the output shaft (wiper and carbon traces), or through another gear driven by the output gear. The output voltage as measured from the center tap of the potentiometer is then used to determine the output shaft position.
Due to the mechanical contact (wiper and carbon traces), the accuracy, stability, and reliability of such potentiometer systems is significantly compromised. In addition, the cost of such systems is still relatively high.
Pulse counting is done through monitoring of DC motor current signals. A sensor is connected to the motor power line to sense the current ripples due to commutation and, with a fixed design, the number of the ripples per shaft revolution is a constant. As a result, the motor shaft position can be determined accurately according to the pulse counting result.
Therefore, accuracy and reliability have become obstacles to development of actuator position control systems, especially for control systems in automotive applications. Accordingly, a low cost solution with a simple circuit and high system efficiency is desirable.